1. Field of the Invention
This invention relates to a magnetic core for a non-contact displacement sensor, and more concretely to a magnetic core for a non-contact torque sensor using a green compact made of an insulated soft magnetic powder being cheap and having a relatively high specific resistance value or so-called powder magnetic core as a skeleton of a coil receiving member constituting the magnetic core and effectively improving mechanical characteristics, which have caused trouble in the use of the green compact, and particularly useful for an electrically operated power steering apparatus for an automobile.
2. Description of Related Art
The power steering apparatus is an apparatus for assisting a steering effort of the automobile. Although a hydraulic power steering apparatus was mainstream in the past, so-called electrically operated power steering apparatus assisting the steering effort by an electric motor is recently and widely developed.
The electrically operated power steering apparatus has various advantages that a controllability is excellent as compared with that of the hydraulic power steering apparatus, and mechanical portions are simple, and a fuel consumption is good, and power is supplied to the electric motor only in case of necessity and the like.
In the system of the electrically operated power steering apparatus, a direction and a quantity of power when operating a handle (a steering wheel) are detected by a torque sensor and a current of the electric motor assisting the steering effort is controlled by a control unit in accordance with the detected value.
In FIG. 7 is shown a main part of a representative non-contact torque sensor constituting the electrically operated power steering apparatus.
The torque sensor 101 is mainly comprised of an input shaft 102 connected to a handle (not shown), an output shaft 103 connected to a side of the steering wheel, a torsion bar 104 connecting the shaft 102 to the shaft 103, first and second detecting rings 105 and 106 arranged on an outer peripheral side of the torsion bar 104 at sides of the input shaft 102 and the output shaft 103 and opposite to each other, and a magnetic core 108 comprising a detecting coil 107 for detecting a relative displacement of the first detecting ring 105 displaced in accordance with a displacement of the torsion bar 104 when a torque is applied to the input shaft 102 with respect to the second detecting ring 106 as a change of an inductance at a non-contact state.
And also, the magnetic core 108 is mainly comprised of the detecting coil 107 wound around a coil bobbin, a toroidal coil receiving member 109 having a space for receiving the detecting coil 107 therein, forming a closed space S together with the detecting coil 107 received therein and retaining the detecting coil 107 in a position separated away at a constant distance from outer circumferential surfaces of the detecting rings 105 and 106, and lead wires 110 connected at their one ends to the detecting coil 107 and led out at the other ends outward from the coil receiving member 109.
In the conventional magnetic core 108, as shown in FIG. 8, the coil receiving member 109 comprises a coil receiving case 111 receiving the detecting coil 107 and a cap 112 press fitted into an opening of the case 111. Each of the coil receiving case 111 and the cap 112 is made of a soft magnetic sintered body formed by compression shaping powder of an electromagnetic stainless steel system or an iron-based soft magnetic material into a green compact and further sintering the green compact. And also, the detecting coil 107 is generally fixed to the inner surface of the coil receiving member 109 with an adhesive 113.
However, since the soft magnetic sintered body is low in the specific resistance value as compared with ferrite, sendust alloy or the like and bad in the detecting sensitivity, if a frequency of a driving power is set to a high value for improving the detecting sensitivity, an eddy current loss becomes large and an inductance lowers, and hence the detecting sensitivity is rather degraded and it is difficult to conduct the detection in a higher accuracy.
On the other hand, as the ferrite and sendust alloy have excellent magnetic properties, they enable the detection in a higher accuracy, but are expensive, so that the application of them to the coil receiving member is unfavorable from a viewpoint of the cost. In addition, the ferrite has a drawback that the temperature change of the inductance is large.
Moreover, the coil receiving member having a relatively high specific resistance value and being cheap is known to be constructed with a green compact made of insulated soft magnetic powder.
The coil receiving member constituted with such a green compact is known to have a good detecting sensitivity because the eddy current loss is small in case of applying a high frequency current. Since the green compact is not subjected to a sintering, however, the coil receiving member is weak in the strength and has particularly a fear of easily breaking when a shock is applied thereto, so that the manufacture is difficult in the conventional press-fitting structure. Furthermore, the applications are critical, and it is said that the application to the electrically operated power steering is difficult in the existing technique.
In the conventional magnetic core (FIG. 8), the detecting coil 107, more concretely, the coil bobbin 114 wound with the detecting coil is merely fixed to the inner surface of the coil receiving member 109 through the adhesive, so that there is assumed a case that the fixing of the coil bobbin 114 to the inner surface of the coil receiving member 109 becomes insufficient. In this case, it is assumed that a strong impact force is applied to the coil receiving member 109 by the coil bobbin 114 to thereby break the coil receiving member 109.
Moreover, nothing is filled in the closed space S formed after the detecting coil 107 is received in the coil receiving member 109, so that there is a possibility of disturbing the wound state of the detecting coil 107, which is in danger of giving a largely bad influence upon the magnetic properties.
Therefore, when the coil receiving member is made of the above green compact, it is required to surely fix the detecting coil 107 to the coil receiving member 109 and to take means for maintaining the wound state of the detecting coil.
And also, a usual displacement sensor is arranged in a housing made of an aluminum alloy or the like in internally contact therewith in order to avoid an influence by an electromagnetic wave from exterior. However, when the coil receiving member not provided on its outer surface with an insulating coating internally contacts with the housing, the eddy current generates in the housing and there is caused a problem of lowering the sensitivity of the detecting coil. In addition, when the housing is made of the aluminum alloy, it is apt to easily cause a phenomenon of biting onto the coil receiving member by a difference of thermal expansion coefficient between the housing and the coil receiving member, especially by a large thermal shrinkage of the housing at a low temperature, and there may be a case that the magnetic core is distorted by such a phenomenon to cause a deviation in the inductance.
It is, therefore, an object of the invention to provide a magnetic core for a non-contact torque sensor using a green compact made of an insulated soft magnetic powder being cheap and having a relatively high specific resistance value as a skeleton of a coil receiving member constituting the magnetic core and effectively improving mechanical characteristics, which have caused trouble in the use of the green compact, and particularly useful for an electrically operated power steering apparatus for an automobile.
According to the invention, there is the provision of in a magnetic core for a non-contact displacement sensor comprising a detecting coil for detecting a relative displacement of a first detecting ring with respect to a second detecting ring as a change of inductance at a non-contact state when the first and second detecting rings are arranged opposite to each other on an outer peripheral side of a torsion bar coaxially connecting an input shaft to an output shaft at sides of the input shaft and output shaft, respectively, and the first detecting ring is displaced in accordance with a displacement of the torsion bar by applying a torque to the input shaft, a toroidal coil receiving member having a space for receiving the detecting coil therein, forming a closed space together with the detecting coil received therein and retaining the detecting coil in a position separated away at a constant distance from outer circumferential surfaces of the detecting rings, and lead wires connected at their one ends to the detecting coil and led out at the other ends outward from the coil receiving member, an improvement wherein the coil receiving member comprises a green compact obtained by compression molding insulated soft magnetic powders into a given shape and an insulating layer covering an outer surface of the green compact, and an organic resin is filled in the closed space of the coil receiving member received with the detecting coil to surely fix the detecting coil in the coil receiving member.
In a preferable embodiment of the invention, the coil receiving member is provided with at least one through-hole for filling the organic resin in the closed space and/or the coil receiving member comprises a coil housing case having upper and lower openings at both ends thereof and capable of housing the detecting coil from the upper opening and laying the detecting coil on the lower opening, and a cap attached to the upper opening of the coil housing case after the housing of the detecting coil to form the closed space.
In addition, a material constituting the insulating layer and the organic resin filled in the closed space are more preferable to be a thermoplastic resin or a thermosetting resin.